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Thread: Complete active noice cancellation

  1. #1 Complete active noice cancellation 
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    I'm interesting to create a portable device which would be able to suppress any
    noices or sound around completely.
    I just wonder if there exist some tone or combination of tones (not necessarely in
    audible range) which would therebly prevent from spreading any other sound waves (at least in audible range)?
    Or create some kind of auditory illusion that person doesn't hear anything?
    Device shoudn't create remarkable sound on it's own.
    Maybe some play with speed of sound could help? :?


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  3. #2  
    Moderator Moderator Dishmaster's Avatar
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    Looking for this?
    http://en.wikipedia.org/wiki/Noise-c...ing_headphones


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  4. #3 Re: Complete active noice cancellation 
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    Quote Originally Posted by Stanley514
    I'm interesting to create a portable device which would be able to suppress any
    noices or sound around completely.
    I just wonder if there exist some tone or combination of tones (not necessarely in
    audible range) which would therebly prevent from spreading any other sound waves (at least in audible range)?
    Or create some kind of auditory illusion that person doesn't hear anything?
    Device shoudn't create remarkable sound on it's own.
    Maybe some play with speed of sound could help? :?
    You have two choices.

    1) Interpose a vacuum between the source and the sender. This is hard to do since shells require some sort of solic connection and it is difficult to breath in a vacuum chamber.

    2) Use a noise canceling method as suggested by Dishmaster. A noise cancelng system, however, only works with more or less steady-state signals. Basically it generates a signal similar to the incoming signal but reveresed in phase. The sum is zero. Because it needs to know something about the incoming signal it has a lag time and does not work well with sharp transients.

    An auditory illusion that a person doesn't hear anything is, however, pretty common. Just observe any teenager being addressed by a parent.
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  5. #4 Re: Complete active noice cancellation 
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    Quote Originally Posted by DrRocket

    2) Use a noise canceling method as suggested by Dishmaster. A noise cancelng system, however, only works with more or less steady-state signals. Basically it generates a signal similar to the incoming signal but reveresed in phase. The sum is zero. Because it needs to know something about the incoming signal it has a lag time and does not work well with sharp transients.
    Fortunately, electricity travels faster than sound, so you can place the listener a little bit closer to the sound source than the speaker which is generating the canceling signal.

    However, it will probably never be perfect. Sound waves expand spherically, so it's very difficult to create a canceling signal that will be in phase with the original signal from every vantage point a listener might try to listen from. The first signal you're trying to cancel has already expanded out to a pretty large spherical area, and you're trying to cancel it by creating a new sphere.
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  6. #5 Re: Complete active noice cancellation 
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    Quote Originally Posted by kojax
    Quote Originally Posted by DrRocket

    2) Use a noise canceling method as suggested by Dishmaster. A noise cancelng system, however, only works with more or less steady-state signals. Basically it generates a signal similar to the incoming signal but reveresed in phase. The sum is zero. Because it needs to know something about the incoming signal it has a lag time and does not work well with sharp transients.
    Fortunately, electricity travels faster than sound, so you can place the listener a little bit closer to the sound source than the speaker which is generating the canceling signal.

    However, it will probably never be perfect. Sound waves expand spherically, so it's very difficult to create a canceling signal that will be in phase with the original signal from every vantage point a listener might try to listen from. The first signal you're trying to cancel has already expanded out to a pretty large spherical area, and you're trying to cancel it by creating a new sphere.
    Noise and vibration cancelling devices work locally. I know of no device that cancels signals globally. The Bose headphones for instance only work for the person wearing them. Active damping devices only work for the device to which they are applied.

    If you put a sensor near the emitting device you run the risk of missing harmonics generated by the environment -- i.e. you may cancel the fundamentals but you will miss the harmonics tha go with echos.
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  7. #6  
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    I've been told that modern surround sound systems initially configure by testing the acoustic peculiarities of a space. Couldn't these systems, with a few microphones, go on compensating dynamically?

    Stanley514 I think your idea has great potential in home entertainment.
    A pong by any other name is still a pong. -williampinn
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  8. #7  
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    Quote Originally Posted by Pong

    Stanley514 I think your idea has great potential in home entertainment.
    I would use it in a nightclub, to create a quiet area for people to talk.

    But the problem is like what Rocket is saying. It would be quite a feat to design a device that could cancel all of the sound in an area.
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  9. #8  
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    Quote Originally Posted by kojax
    Quote Originally Posted by Pong

    Stanley514 I think your idea has great potential in home entertainment.
    I would use it in a nightclub, to create a quiet area for people to talk.

    But the problem is like what Rocket is saying. It would be quite a feat to design a device that could cancel all of the sound in an area.
    And if you did there would not be much conversation in that area. Now you are talking about canceling only selected sound, not all the sound. That is really hard.
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  10. #9  
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    True. However cancelling everything but known audio coming from a TV is easier isn't it?

    Think of this as enhancing the quality of digital audio experience, within the envelope of a home entertainment setup.

    (Think $$$ )
    A pong by any other name is still a pong. -williampinn
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  11. #10  
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    This is actually an underestimated problem in more households than you'd expect. People don't always connect the boxes correctly to their stereo, and complain that the music sounds terrible, or the volume is not what they expected, while they are involuntarily cancelling part of it. In stereo, this can give strange effects such as the text in a song being hard to hear, while the music plays normal (voice signal is usually the same for both channels IIRC).
    My parents in law almost thought their boxes where broken after cleaning the house last year.
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  12. #11  
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    Quote Originally Posted by Bender
    This is actually an underestimated problem in more households than you'd expect. People don't always connect the boxes correctly to their stereo, and complain that the music sounds terrible, or the volume is not what they expected, while they are involuntarily cancelling part of it. In stereo, this can give strange effects such as the text in a song being hard to hear, while the music plays normal (voice signal is usually the same for both channels IIRC).
    My parents in law almost thought their boxes where broken after cleaning the house last year.
    I think you are describing a problem with the emission of the sound as opposed to acoustic wave effects caused by reflections in the room. If so, that would indeed produce poor sound quality, but I would not call it noise cancellation. It sounds more like trying to play the bass through the tweeter or something along that line.
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  13. #12  
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by kojax
    Quote Originally Posted by Pong

    Stanley514 I think your idea has great potential in home entertainment.
    I would use it in a nightclub, to create a quiet area for people to talk.

    But the problem is like what Rocket is saying. It would be quite a feat to design a device that could cancel all of the sound in an area.
    And if you did there would not be much conversation in that area. Now you are talking about canceling only selected sound, not all the sound. That is really hard.
    Oh yeah. My goal would be just to silence the music in that area, and leave the rest alone. Just imagine how cool it would be. You're still in the same room as the massively blairing music, but it suddenly gets quiet as you walk past the silencing machinery. You're free to sit down, and have a calm, quiet conversation with the young lady accompanying you, while you watch the others go on dancing.

    (The last bit might be wishful thinking on my part... :wink: )
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  14. #13  
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    Hearing Aids plus FM system, it works pretty well for me.
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  15. #14  
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    Quote Originally Posted by kojax
    Oh yeah. My goal would be just to silence the music in that area, and leave the rest alone. Just imagine how cool it would be. You're still in the same room as the massively blairing music, but it suddenly gets quiet as you walk past the silencing machinery. You're free to sit down, and have a calm, quiet conversation with the young lady accompanying you, while you watch the others go on dancing.

    (The last bit might be wishful thinking on my part... :wink: )
    Depending on the size and shape of the room, putting an extra box in counterphase pointing from the room to that corner could already reduce the noise. Barely more difficult is put a microphone right in front of the extra box and connect that signal in counterphase to the box. These simple solutions work best for lower sound frequenties: 300 Hz sound has a wavelength of approximately 1 m, 3000 Hz sound has a wavelength of approximately 10 cm, so it's already more difficult to aim in the right phase.
    For better performance, you should build in the right delay, or better: a tuneable delay.

    But often in a bar of club you already have more quiet places. e.g. in the corners next to a stage you're usually next to the boxes or even a bit behind them. You could simply put a box a few meters past a corner instead of in a corner pointing to the room, and that corner will be a lot quieter.
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